Flywheel-based energy storage on a heave-compensating drawworks

ABSTRACT

A system for managing energy consumption in a heave-compensating drawworks includes a power supply, a winch drum connected to the power supply so as to receive power from the power supply, a flywheel connected to the winch drum and to the power supply, and a controller connected to the power supply and to the winch drum for passing energy to and from the flywheel during an operation of the winch drum. The flywheel includes a disk rotatably coupled to an AC motor. The power supply includes a first pair of AC motors operatively connected on one side of the winch drum and a second pair of AC motors operatively connected on an opposite side of the winch drum.

RELATED U.S. APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to offshore drilling activities. Moreparticularly, the present invention relates to offshore drawworks thatinclude heave compensators so as to cause the drill string to move inrelation to the heave of the vessel upon which the drawworks is located.Additionally, the present invention relates to flywheels that can beused for energy storage and used, in particular, in association with thecyclic loads.

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 37 CFR 1.98

There are many systems in current use that have a high power consumptioncycle and a low power consumption cycle. These systems can includecranes, drag lines, oil well derricks, swell compensators, hopper armgimbals and drag head controls. Quite often, in the oil productionindustry and in mining operations, it is quite common to require highpower and energy consumption during certain portions of the operatingcycle and low power consumption during another part of the operatingcycle of the system. As an example, a crane used in either offshoreoperations or in mining operations will require very little power to themotor of the crane during the lowering of the bucket of the crane. Incontrast, a great deal of power is required by the motor in order tolift a loaded bucket from a lower position to a higher position.

Conventionally, in such cyclic operations, it is necessary to size thepower supply so as to accommodate the maximum expected power consumptionduring the high energy consumption cycle of the operation. During thecyclic loads, the power supply will continually cycle between thedelivery of maximum power and the delivery of minimum power. It has beenfound that the cyclic loads on the power supply causes adisproportionately large amount of fuel consumption and, accordingly,adverse environmental impacts.

One particular type of cyclic operation occurs in heave-compensationdrawworks. Typically, these heave-compensation drawworks will be placedupon a drilling ship. The drilling ship is utilized so as to drill foroil and gas in offshore operations. In these drilling ships, the drillstring will extend through a floor of the ship and be supported by awire rope connected to a sheave system. A winch drum is connected to anend of the wire rope so as to pay out and pay in the drill stringrelative to the wave action affecting the drilling ship. Since it isimportant to maintain proper weight-on-bit during the drillingoperation, the heave-compensation system is relatively complex. Forexample, when there is an upward heave of the drilling ship, the winchdrum should pay out the wire rope so as to maintain the drill string ina proper location below the ship and to maintain proper weight-on-bit.On the other hand, when there is a downward movement of the ship, forexample, by a trough of the waves, then the winch drum will pay in thewire rope so as to prevent the drawworks from exceeding the properweight-on-bit from the downward force caused by the downwardly movingdrill ship. Typically, on these heave-compensation drawworks, the motorsthat are associated with the winch drum and the drawworks can only lowerthe drill string at a maximum predetermined rate. Preferably, the drillstring should be lowered as quickly as possible. However, because of theinertia associated with each of the motors of the drawworks, the motorsmust be controlled so as to prevent the maximum rate of downwardmovement of the drill string. As such, energy consuming actions, such asthe application of braking forces, are placed upon the motors associatedwith the winch drum even during relatively non-energy consumingactivities, such as the lowering of the drill string.

When the drill string is being raised for various purposes, the motorsmust exert sufficient power so as to elevate the drill string at adesired rate. In certain circumstances, the drill string must be liftedso as to allow for the replacement of the bit. This requires a greatdeal of energy consumption since the entire weight of the drill stringmust be lifted. As such, the motor requirements for the drilling shipare particularly high since the motors must be sized so as to be able tolift a great deal of weight associated with the drill string. A problemis that when large motors are used for the lifting of the drill string,greater braking capacity is required since large motors will havegreater inertia during the lowering of the drill string.

In the past, DC motors have been used for the paying in and out of thewire rope on such heave-compensation drawworks. These DC motorstypically will require a transmission so as to carry out the properraising and lowering activities. The DC motor is clutched out during thepaying out of the wire rope. A friction-type brake is utilized so as toprevent excess speed and to prevent excess inertia of the DC motor.These friction-type brakes have included, in the past, eddy-currentbrakes and disk brakes. Typically, the eddy-current brake is attached tothe drive of the drawworks. Whenever disk brakes are used, they willtend to wear out over time.

Recently, AC motors have been incorporated into drilling ships for thepurposes of controlling the drawworks and for the operation of the winchdrum. These AC motors offer the benefit of greater torque and a fixedgearbox ratio. These AC motors do not require clutches. However, theywill have a restricted pay out speed. Typically, the AC motor itself isused for the braking of the motor inertia. In the past, dynamic brakingresistors have been employed with the use of the AC motors so as tocapture some of the braking energy.

Unfortunately, these dynamic braking resistors accumulate excess energywhich needs to be burned off. In offshore facilities, this excess energyis often used for the hotel load of the facility. However, offshoreoperators often struggle to find extra utilities to burn off the excessenergy. In the past, it has been found that this excess energy from thedynamic braking resistors can be applied to the thrusters associatedwith the drilling ship. In particular, the engines associated with thethrusters are powered and operated by this excess energy. When theexcess energy becomes too great, then drill ship operators will oftenpoint the thrusters at each other so as to maintain a stable position inthe water while burning the excess energy. Unfortunately, the use of theenergy in this manner will tend to quickly burn out the enginesassociated with the thruster and possibly compromise the DP 2classification of the offshore system. In offshore facilities, such asdrilling ships, the loss of the DP 2 classification is critical to theoffshore operator. In the event that the ship does not have the abilityto properly control its position relative to the bore hole, then therecan be severe repercussions associated with the loss of position. Assuch, all drilling ships must maintain the redundant capability of itsengines and the ability to maintain position within the water.

In the past, various flywheel systems have been utilized for the controlof energy loads. U.S. Pat. No. 5,712,456, issued on Jan. 27, 1998 toMcCarthy et al., describes a flywheel energy storage system foroperating elevators. The elevator system, having a three-phase rectifierwhich converts energy to a three-phase AC main to provide DC power on abus to a three-phase inverter that drives a three-phase inductive hoistmotor, utilizes the generated energy applied to a boost regulator todrive a flywheel motor generator to store the regenerated energy in theform of inertia therein. When the flywheel motor generator reaches alimiting speed, any continued regenerated energy is dumped in an energydissipating device. During periods of high demand, the inertial energystored in the flywheel generator is used to add energy to the DC bus toprovide additional current to the three-phase inverter for driving thehoist motor. The control is provided by software embedded in a elevatorcomputer.

U.S. Pat. No. 6,043,577, issued on Mar. 28, 2000 to Bornemann et al.,describes a flywheel energy accumulator having a vertical shaftrotatably supported in a vacuum housing by superconductive magneticaxial support bearings. Lower and upper flywheels are mounted on theshaft in axially spaced relationship. A homopolar dynamic machine with arotating magnetic field is disposed in the space between the flywheelsand includes a stator supported in, or forming part of, the housing. Arotor is mounted on the shaft.

U.S. Pat. No. 6,172,435, issued Jan. 9, 2001 to J. Tanaka, teaches aflywheel power source device for converting electric energy into kineticenergy and for storing the kinetic energy by rotating a flywheel. Theflywheel is supported by a rotary shaft that is rotatably mounted in abearing in a casing. The kinetic energy is reconverted into electricenergy when necessary.

U.S. Pat. No. 6,236,127, issued on May 22, 2001 to Bornemann, describesanother type of flywheel energy accumulator that has a vertical shaftwith the rotor of an electric motor/generator in a vacuum-type housing.Flywheels are mounted on the shaft at opposite sides of the rotor. Theelectric motor/generator and the flywheels are placed in modules whichare mounted on top of one another.

U.S. Pat. No. 6,365,981, issued on Apr. 2, 2002 to M. Tokita, provides apower generation system with a flywheel apparatus. The flywheelapparatus has a frame, a flywheel section and an exciting section. Theflywheel section has an input unit having the input shaft, first andsecond flywheel units having the output shaft, and first and seconddrive units for transmitting the rotary force of the input unit to thefirst and second flywheel units. The exciting section increases theflywheel effect of the flywheel section.

U.S. Pat. No. 6,819,012, issued on Nov. 16, 2004 to C. W. Gabrys,discloses a flywheel energy storage system that has an energy storageflywheel supported in a low pressure containment vessel for rotation ona bearing system. A brushless motor/generator is coupled to the flywheelfor accelerating and decelerating the flywheel for storing andretrieving energy. The flywheel is rotated in normal operation at aspeed such that the generator voltage is higher than the output voltage.Power supplied to the load from the generator is a regulated output thatis maintained at a substantially constant voltage level by usingswitching regulation of the alternating current voltage generated by thegenerator. The switching regulation of each generator phase occurs at afrequency equal to or less than twice the frequency of the generatoralternating current. As so operated, the flywheel uninterruptible powersupply efficiently maintains power to an electrical load during aninterruption of primary power by supplying power generated from theflywheel generator.

U.S. Pat. No. 7,078,880, issued on Jul. 18, 2006 to Potter et al.,provides an energy storage flywheel voltage regulation and load sharingsystem. This system for regulating the voltage in an electricaldistribution system includes a plurality of flywheels, motor/generators,and controllers. Each of the motor/generators is coupled to one of theenergy storage flywheels and to the electrical supply system. Themotor/generators each supply one or more signals representative ofmotor/generator operational parameters, and each motor/generatorcontrollers receive one or more of the motor/generator operationalparameter signals from each of the motor/generators. In response to theoperational parameter signals, the motor/generator controllers eachcontrol the operation of one of the motor/generators in either a motormode or a generator mode. This regulates the electrical supply systemvoltage and equally shares the electrical load between themotor/generators.

It is an object of the present invention to provide an energy storagesystem on a heave-compensation drawworks that effectively stores,absorbs and relinquishes energy.

It is a further object of the present invention to provide an energystorage system on a heave-compensation drawworks that eliminates theneed for transmissions.

It is a further object of the present invention to provide an energystorage system on a heave-compensation drawworks that decreases powerconsumption requirements.

It is still another object of the present invention to provide an energystorage system on a heave-compensation drawworks that achieves arelatively constant load profile free of peaks and valleys from thepower source.

It is another object of the present invention to provide an energystorage system on a heave-compensation drawworks which achieves thefastest drop speed possible.

It is another object of the present invention to provide an energystorage system on a heave-compensation drawworks which avoids the needfor brakes.

It is still a further object of the present invention to provide anenergy storage system for use on a heave-compensation drawworks thatmaximizes fuel savings while minimizing emissions.

It is still a further object of the present invention to provide anenergy storage system on a heave-compensation drawworks that extendsengine life.

It is still another object of the present invention to provide an energystorage system on a heave-compensation drawworks that avoids the use ofbatteries and dynamic braking resistors.

These and other objects and advantages of the present invention willbecome apparent from a reading of the attached specification andappended claims.

BRIEF SUMMARY OF THE INVENTION

The present invention is a system for managing energy consumption in aheave-compensation drawworks. This invention comprises a power supply, awinch drum connected to the power supply so as to receive power from thepower supply, a flywheel connected to the winch drum and to the powersupply, and a control means connected to the power supply and to thewinch drum for passing energy to and from the flywheel during anoperation of the winch drum.

In the present invention, the flywheel includes a disk coupled rotatablyto an AC motor. In particular, in the preferred embodiment, the flywheelhas a first AC motor and a second AC motor facing the first AC motor.The disk is positioned between the first and second AC motors.

The power supply of the present invention is an AC motor mounted on oneside of the winch drum. In the preferred embodiment of the presentinvention, this power supply includes a first pair of AC motorsoperatively connected on one side of the winch drum and a second pair ofAC motors operatively connected on an opposite side of the winch drum.The first and second pairs of AC motors can be coupled to the flywheelby a common shaft. In the preferred embodiment of the present invention,the AC motor is a dual stator induction motor.

The system of the present invention includes a wire rope wound aroundthe winch drum, a sheave receiving the wire rope thereover, and a hookload affixed to the end of the wire rope opposite the winch drum andbelow the sheave. The wire rope extends and retracts relative to arotation of the winch drum. The equipment of the present invention canbe placed upon a drilling ship having the power supply, the winch drum,the flywheel, and the control means positioned thereon. A gear reductioncan be interposed between the power supply and the winch drum.

The present invention is also a process of managing energy consumptionin a heave-compensation drawworks. This process includes the steps of:(1) rotating the winch drum so as to pay out the wire rope and to lowerthe drill string downwardly; (2) transferring the energy from thelowered drill string to a flywheel so as to rotate the flywheel; and (3)transferring energy from the rotating flywheel to another location onthe drawworks.

The step of transferring energy from the rotating flywheel includestransferring energy from the rotating flywheel to the winch drum andraising the drill string by paying in the wire rope over the winch drum.An AC motor is connected to the winch drum so as to apply rotationalforces to the winch drum. There is another AC motor connected to theflywheel.

The step of transferring energy from the lowered drill string includestransferring energy from the AC motor connected to the winch drum toanother AC motor connected to the flywheel. In the process of thepresent invention, the process is used for compensating for heave of thedrawworks by rotating the winch drum in relation to a movement of thedrawworks in response to wave motion. The step of connecting an AC motorincludes coupling the winch drum to a first AC motor by a shaft on oneside of the winch drum and coupling the winch drum to a second AC motorby the same shaft on an opposite side of the winch drum. The heave iscompensated by moving the sheave assembly vertically in relation to amovement of the drawworks in response to wave motion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of the system and process of thepresent invention.

FIG. 2 is a block diagram showing the processing of energy in accordancewith the present invention.

FIG. 3 is an illustration showing the use of the AC motors inassociation with the winch drum and the flywheel.

FIG. 4 is a graph showing the relationship of hoist capacity of thepresent invention relative to the prior art.

FIG. 5 is a graph illustrating heave amplitude in relation to hook load.

FIG. 6 is a graphical illustration of the active heave profileassociated with the present invention relative to the prior art.

FIG. 7 is a fuel consumption histogram showing the relationship of fuelconsumption by the present invention relative to the prior art.

FIG. 8 is a graph showing average fuel consumption by the presentinvention in relation to the prior art.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a drilling ship 10 as illustrated asfloating on a body of water 12. The drilling ship 10 is utilized inassociation with heave-compensating drawworks 14. As can be seen, aderrick 16 extends upwardly from the deck 18 of ship 10. A sheaveassembly 20 is positioned at the top of the derrick 16. The sheaveassembly 20 supports a wire rope 22 extending thereover. Wire rope 22extends downwardly so as to support a hook load 24 at an end of the wirerope 22. The hook load 24 is configured so as to support the drillstring 26. It can be seen that the drill string 26 extends to the floor18 of the ship 10 and downwardly through the body of water 12.Ultimately, the drill string 26 will have a bit on the end opposite thehook load 24 which is used for the drilling activities. It is importantto note that the weight supported by the hook load 24 and by the wirerope 22 is extremely heavy. A winch drum 28 is connected to the oppositeend of the wire rope 22 and supported on the deck 18 of the ship 10. Thewinch drum 28 is used for paying in and paying out the wire rope 22 inrelation to the desired movement of the drill string 26 or the desiredweight-on-bit at the end of the drill string 26.

Since the water 12 will have wave action thereon, the ship 10 will tendto follow this wave action and heave upwardly and downwardly. Since itis desired to maintain a relatively constant weight-on-bit inassociation with the drilling activities of the drill string 26, thesheave assembly 20 and the associated wire rope 22 will need to raiseand lower in relation to the wave motion of the water 12. When a wave isencountered and a ship elevates by reaching the crest of the wave, thenthe sheave assembly 20 and the associated wire rope 22 should be payedout by the winch drum 28 so as to maintain the drill string 26 at aconstant position. On the other hand, when the ship 10 encounters atrough of the wave of water 12, it will be necessary for the winch drum28 to rotate to draw in the wire rope 22 and to raise the sheaveassembly 20 in order to maintain this proper position of the drillstring 26. The hook load 24 can also be used for the lowering of thedrill string 26 prior to drilling. Under these circumstances, the winchdrum 28 will pay out the wire rope 22 so as to properly lower the hookload 24 for the purposes of lowering the drill string 26 to a desiredunderwater location.

In the prior art, when the drill string 26 is lowered to its underwaterlocation, the winch drum 28 would need to be braked so as to preventundesired inertia. As such, the maximum speed of lowering of the drillstring 26 was precisely controlled. It was not possible to achieve themaximum lowering rate because of the concern for the motor inertia andthe need for braking.

FIG. 2 illustrates the energy control system 30 in accordance with theteachings of the present invention. There is shown in block 32 that thehook load 34 is supported on a wire rope 36 extending from sheave 38. Apower supply 40 provides energy for the controller 42. The power supply40 can be a utility grid 44 or an AC motor 46. Power is delivered fromthe power supply 40 along line 48 to the controller 42. Controller 42 isalso connected to a flywheel energy storage system 50. The flywheelenergy storage system includes a flywheel 52 and an AC motor 54. A powersink 56 is connected by line 58 to the controller 42. Power sink 56 canbe in the nature of batteries at another location on the drawworks. Thecontroller 42 includes a suitable algorithm so as to direct energy toand from the various blocks 32, 40, 50 and 56 in relation to the powerdemands of the heave-compensation drawworks.

FIG. 3 is an illustration of the operation of the flywheel and winchdrum of the present invention. Initially, it can be seen that the winchdrum 60 has wire rope 62 wound therearound. The winch drum 60 iscontrollably connected to AC motors 64, 66, 68 and 70. A common shaft 72will extend between the AC motors 64, 66, 68 and 70 and be connected tothe winch drum 60. As such, the AC motors 64, 66, 68 and 70 can deliverthe requisite power for the rotation of the winch drum 60 in accordancewith the requirements of the heave-compensating drawworks. The power ofthe AC motors 64, 66, 68 and 70 can be delivered by line 82 (or anyother conduit) to the AC motors 74 and 76 associated with a flywheel 78.A common shaft 80 joins the AC motors 74 and 76 to the flywheel 78. Theflywheel 78 is in the nature of a steel disk or a disk assembly. As usedherein, the flywheel 78 can also be incorporated onto common shaft 72 inany position between any of the AC motors 64, 66, 68 and 70 within theconcept of the present invention.

In experiments conducted with the present invention, it is found thatthe present invention achieved many advantages over the prior art. Theenergy storage system of the present invention is particularly adaptedfor use with the drawworks-based heave-compensation of a deep-seadrilling vessel. This is because there are highly predictable and periodcharacteristics of ocean swells that readily allow for highly efficientenergy recovery. As such, the present invention eliminates the need forwasteful dissipation schemes.

Studies of the present invention are based upon an actual drawworks(HITECH AHC-1000) a one-thousand ton drawworks, in operation on a drillship. The system that was analyzed includes the AC motors 64, 66, 68 and70 with suitable gear reductions, the winch drum 60 and the windingmechanics, the wire rope 62, the sheave assembly 20 and the hook load24. The systems are separately modeled and coupled via force balances.

The HITEC AHC-1000 drawworks uses six GEB22 AC induction motors. Theyconvert electrical power into a mechanical torque directly applied to arotating inertia (the rotor) at a constant efficiency of 95%. The motorsand drives are able to fully regenerate power at an efficiency ofapproximately 95%. The motors are speed-controlled via PID controllers.The outputs are limited by speed-torque characteristics and major driveparameters (slew rate, speed limits, etc.). Due to the periodic natureof heave compensation, the motors are allowed run at 140% load whileheave compensating.

In particular, dual-stator AC motors are used as the AC motorsassociated with the winch drum. These motors are large, low speed, veryhigh torque machines that are directly coupled to the winch drum. Assuch, it eliminates the necessity for a gear reduction. The rotor isring-shaped and has a concentric outer stator and inner stator. Theouter stator delivers 68% of the total torque, while the inner statordelivers the remaining 32%.

The drawworks requires four such dual-stator AC motors, such as motors64, 66, 68 and 70. Motors 64 and 66 are located on one side of drum 60while the motors 68 and 78 are located on the opposite side of drum 60.These motors 64, 66, 68 and 70 are coupled to each other by a commonshaft 72. Since rotational inertia on the high speed side of a gearreduction is effectively multiplied by the square of the gear ratio,this direct drive setup will have a significantly lower total inertia(approximately 35% lower than the GEB22 equipped system) while theoverall weight stays roughly the same. This system will also benefitfrom the elimination of gear backlash, greater redundancy, a lower costdrive system, and high breakdown torque (225% of nominal).

The torque characteristics of the HITEC AHC-1000 ton drawworks is shownhere below in Table 1:

TABLE 1 Symbol Value Unit Drawworks specs Gear ratio GR 10.5 Number ofline parts N 14 Drum radius R 1.867 M Drum length L 2.057 M Drum inertiaI_(d) 12000 kg · m² Wire rope diameter 2Rear wheel 50.8 Mm Wire ropeelastic E 90 GPa modulus GEB22 AC induction motor Nominal speed 800 RPMContinuous torque T_(c) 10,260 N.m Continuous power P_(c) 858 kWIntermittent overload 140 % capacity Intermittent torque Tj 14,364 N.mIntermittent power P_(i) 1,201 kW Inertia Im 18.2 kg · m² Number ofmotors Nm 6

The direct drive drawworks of the preferred embodiment of the presentinvention is particularly described in Table 2 hereinbelow:

TABLE 2 Symbol Value Unit Drawworks specs Gear ratio GR 1 (direct drive)Number of line parts N 14 Drum radius R 1.867 M Drum length L 2.057 MDrum inertia I_(d) 12000 kg · m² Wire rope diameter 2.Rw 50.8 Mm Wirerope elastic E 90 GPa modulus High torque/dual stator AC induction motorNominal speed 98 RPM Continuous torque T_(c) 155,900 N.m Continuouspower P_(c) 1600 kW Intermittent overload 150 % (conservative capacityrating) Intermittent torque T_(i) 233,900 N.m Intermittent power P_(j)2400 kW Inertia I_(m) 1950 kg · m² Number of motors N_(m) 4

For proper sizing of the flywheel 78, an estimate should be made of thedrawworks' peak power draw and energy fluctuation. It is proper toconsider the potential (block height relative to ship deck) and kinetic(rotation of drum and motor) energy contained in the drawworks system.This is calculated in accordance with the following formulas:

$E = {{\frac{1}{2}I_{t}{\overset{.}{\theta}}^{2}} - {{MgH}(t)}}$where${\overset{.}{\theta} = {\frac{N}{R}\frac{{H(t)}}{t}}},{{H(t)} = {A\; {\sin ( {\frac{2\pi}{P}t} )}}},{I_{t} = {{I_{m}{GR}^{2}} + I_{d}}}$

In this approximation, the hook is assumed to be held absolutelymotionless while the ship heaves sinusoidally with heave height H(t).Energy is contained as potential energy in the relative hook height andas kinetic energy in the rotating motors and winch drum. Drum speed isassumed to be exactly proportional with heave speed, with a factor basedon the number of line parts N and drum radius R. The total effective{dot over (θ)} inertia I_(t) stems from the drum inertia plus the totaleffective inertia of the motors as seen through a reduction of gearratio GR. Using the following equations, the absolute minimum andmaximum of E(t), and the total (peak-to-peak) energy fluctuation asshown herein below:

Δ E = 2 C₂  if  C₂ > 2 C₁${\Delta \; E} = {{C_{1} + \frac{C_{2}^{2}}{4\; C_{1}} + {C_{2}\mspace{14mu} {if}\mspace{14mu} C_{2}}} \leq {2\; C_{1}}}$where $C_{1} = \frac{2\pi^{2}N^{2}A^{2}I_{t}}{R^{2}P^{2}}$ C₂ = MgA

Using these equations, and taking the torque and power limits intoaccount, an estimate can be made as to the maximum energy fluctuations.Since the energy fluctuation can become arbitrarily large withincreasing heave period (allowing high amplitude, but low-speed heavingmotion), an upper limit to the heave period must be set. This limit ischosen as 18 seconds, corresponding to the maximum considered in theHITEC AHC-1000 specification. For an 18 second period, the maximumenergy fluctuation will be about 41 MJ. By using 6 motors at 140%capacity and 95% efficiency, the maximum power draw will beapproximately 7600 KW.

The flywheel system 78 is a very simple system. It is a simple torquedevice (an AC induction motor/generator coupled to a large rotationalinertia). The governing equation is:

T=I{dot over (ω)}+T _(d)

where T is shaft torque, T_(d) is aerodynamic drag torque, I is therotational inertia and {dot over (ω)} denotes the time derivative ofangular velocity. The total amount of kinetic energy contained in therotating mass is:

E=½Iω²

and the power transfer at the time is derivative of this. Theaerodynamic drag of the spinning flywheel is estimated by consideringthe shear drag on a flat plate, aligned parallel to a fluid stream:

F_(plate)=½C_(Df)ρAV²

where F_(plate) is the drag force, C_(Df) is the shear drag coefficient,ρ is fluid density A is the reference area, and V is linear velocity. Byassuming that the flywheel is cylindrical with a thickness D, the aboveequation is integrated over the entire surface of the flywheel. As such,the total drag torque is:

T_(d) = ∫_(S)∫F_(plate)rA = πρ C_(Df)ω²(2/5 r⁵ + Dr⁴)

A number of empirical formulas already exist to determine the dragcoefficient. In the present case, the following formula for turbulentflow is used:

$C_{Df} = \frac{0.455}{( {\log ({Re})} )^{2.58}}$

where the Reynolds number R_(e) is based on the flywheel radius and tipspeed. The specifications of the flywheel are identified hereinbelow inTable 3:

TABLE 3 Motor/aenerator (2 per flywheel) Nominal speed 800 RPM Nominalpower 746 kW Intermittent power 1,201 kW Flywheel Diameter 1.9 mThickness 0.20 m Inertia 2000 kg · m² Weight 4555 kg Speed range500-1650 RPM Energy capacity 28 MJ Power dissipation @ max 71 kW speed

The flywheel system has two main goals. First, it should store and reuseregenerated power so as to realize a lower overall average powerconsumption. Second, it should buffer the equipment's power requirementsin such a way that the power source sees a relatively constant loadprofile free of extreme peaks and valleys. It is desirable to dimensionthe device such that its energy and power capacity is consistent withthe machine's maximum estimated peak-to-peak energy fluctuations andpower demand. Since it has been estimated that the energy fluctuation isestimated to be approximately 41 MJ, a flywheel capacity of roughlydouble that amount will ensure a sufficient energy range. The maximumpower to be delivered by the flywheel is roughly equal to the maximumpower rating of the equipment it will be used with. In particular, theflywheel system can consist of the same GEB22 AC motors as used in thedrawworks. The flywheel will include a large steel disc. Motors 74 and76 face each other so as to drive a flywheel 78 situated in betweenthem. Three of these units are sufficient to provide the power andenergy storage capacity required for the heave-compensating drawworks.The motors all operate in parallel.

Relative to the FIG. 2, it can be seen that there are four main powergenerating or consuming systems located on the drilling ship 10 andassociated with the heave-compensating drawworks. The main power source,as shown in block 40, is used for power generation. The equipment, asshown in block 32, is the drawworks system. This equipment 32 canprovide power consumption and energy generation or regeneration. Theflywheel energy storage system, as illustrated in block 50, providespower consumption and energy generation or regeneration. Finally, thereis the excess power sink, as illustrated in block 56, which providessolely power consumption. The controller 42 utilizes a flywheel controlalgorithm. The controller 42 will coordinate energy transfer between thesystems. The controller 42 routs power to and from the blocks 32, 40, 50and 56 according to the following priorities (in order of importance):(1) the power demand of the equipment in block 32; (2) the flywheelenergy storage system 50 operating within its preset limits; (3) thepower drawn from the main power source 40 is constant; (4) the flywheelenergy storage system 50 contains sufficient energy to supply the nextdemand peak; (5) the flywheel energy storage system 50 has sufficient“headroom” to absorb the next regeneration peak; and (6) a minimalamount of power is routed to the power sink 56. There are severalpossible routes along which the controller 42 can transfer energy. Foreach route, an energy index (I_(E)) and a power index (I_(P)) isdefined. The energy index is related to the flywheel charge and thepower index is related to the power being demanded by the application.The amount of power to be transferred along each route is a function ofthese two indices. Graphically, a surface in three-dimensional space isdefined for each route. The value of I_(E) and I_(P) can be seen ascoordinates defining a point on this surface. The height of the surfaceat this point is then a measure of the amount of power to be transferredalong that route. These surfaces are chosen so that the prioritieslisted above are satisfied.

As an example, there is a route for controlling the amount of powerrouted from the equipment 32 to the power sink 56. In this case I_(E)and I_(P) are directly linearly related to the flywheel charge and thepower demand of the equipment 32. The associated surface has zero heightalmost everywhere. In other words, when power demand is negative (thedrawworks is regenerating) and the flywheel is almost fully charged, thesystem will only begin to dissipate power through the sink 56.

The power and energy indices can be directly linearly related to thecurrent power demand or flywheel charge, respectively, or they can befiltered in some way. For example, the power transfer is scaled by afactor obtained by passing the power demand signal of the equipment 32through a low-pass butterworth filter with a cutoff frequency below themachine's typical operating cycle frequency. The surface is shaped sothat the power is continuously routed at a fairly constant trickle fromthe power supply 40 to the flywheel energy storage system 50. This willoccur at approximately the rate of the machine's average powerconsumption except for when the power demand is high and flywheel isalmost depleted (i.e. where power generation resources are divertedstraight to the equipment 32) and when the flywheel energy storagesystem 50 is near maximum charge.

Besides the main control system 42, some minor logic can be added tofurther optimize the system's performance. Two such additions are a gridpeak limiter, which limits the maximum power draw from the grid to apreset value and a precharge unit which precharges the flywheel tocompensate for initial filter start-up transients. The system is alsoeasily adaptable to other applications. The most important parameters toconsider are those for the low-pass filters (filter order and cut-offfrequency). The power routing surfaces as developed for this system aregenerally suitable to most cyclical applications.

The simulations conducted with these systems were developed in and runin Simulink. This is a software package that is used in conjunction withMATLAB™. As such, it provides a graphical interface to model highlycomplex dynamic systems in the form of familiar block diagrams. Thesimulation is divided into two separate parts. First, the drawworksdynamics simulation is run for a certain amount of time This simulationoutputs, among other things, the drawworks' power requirement loadprofile. This load profile is subsequently used as an input for thesimulation of the flywheel dynamics and the flywheel control system. Themathematical nature of the drawworks dynamics simulation is differentfrom that of the flywheel control system. The drawworks model is a“stiff” nonlinear differential equation. This means that the solutioncan sometimes change very abruptly on a time scale that is very shortcompared to the time scale of interest. For example, when a new layerforms on the winch drum 60, the velocity of the wire rope 62 changesvery abruptly. As such, it needs to be calculated in simulation timesteps in the order of microseconds. The flywheel control systemsimulation contains many logic-based components that can change theiroutput between discrete states instantaneously. The flywheel controlsystem is simulated using a fixed-step discrete solver.

Before the simulation is started, a parameters file is provided whichincludes simulation settings (solver, step time, simulation duration),drawworks parameters (inertias, geometry, speed-torque characteristics,etc.) and flywheel control system settings (power routing surfaces,filter settings, etc.). First, the drawworks simulation run and itspower demand output is loaded into the flywheel simulation as an input.The most important output of the flywheel simulation is the system'stotal power draw. This output and others are used in the post-processingwhere resulting electric power draw and diesel generator set fuelconsumption and emissions are calculated and plots of this data aregenerated.

FIG. 4 illustrates the hoisting capacity verification. As can be seen,for an intermittent hoist the hook load can be at its maximum and willdecline relative to the block speed. In other words, the amount of hookload in an intermittent hoist is reduced relative to the desired blockspeed. On a continuous hoist or a continuous hold, the maximum hook loadis less than for the intermittent hoist and will decline at an earlierpoint than that for an intermittent hoist.

FIG. 5 shows the heave-compensation limits calculated through simulationand overlaid onto the scanned AHC 1000 plot. The areas where the resultsdiverge slightly from the specifications can be attributed to differingfriction factors, motor specifications and control parameters. As such,it can be seen that the hook load for the systems is rather inverselyrelated to heave amplitude. Where the heave amplitude is great, thenless hook load is possible. However, when heave amplitude is minimal,then a large hook load can be achieved.

The simulation was run for several different limit cases for both theoriginal HITEC AHC-1000 and direct drive configurations so as to ensurethat the system performs adequately under different conditions. Theoverall power draw profile for the drawworks system is significantlyimproved when the flywheel system is implemented. This is illustrated inFIG. 6. The high peaks (which coincide with the downward heavingmovement) and deep valleys (upward heave) are fully buffered by thestored energy in the flywheels. The resulting power draw profile showsonly minor fluctuations. FIG. 6 illustrates the effect of the flywheelsystem's start-up transient. FIG. 6 plots the total external power drawof the drawworks. Initially, the flywheel's precharge is supplying mostof the power to the drawworks, while external power draw slowly picksup. It can be seen that the peak load is reduced by a factor of 10 ormore in most cases while average power draw is reduced approximately bya factor of 4. At low hook loads, the effect of the lower rotationalinertia of the direct drive system can be seen in the load profile ofFIG. 6. At higher hook loads, this effect is overshadowed by the powerdemanded by the linear heaving motion.

The drastic reduction in peak power draw means that power productioncapacity can be reduced, for example, by putting generator sets offline.As a hypothetical example, we assume that the drawworks is powered by anumber of CAT 3516 1280 KW diesel generator sets. During active heavecompensation operations, in four meter peak-to-peak waves, with a periodof 18 seconds and a 900 ton hook load (the bottom plot in FIG. 9) six ofthese gensets would normally need to be online. With the energy storagesystem in use, only a single genset needs to be online due to thegreatly reduced peak power draw.

FIG. 7 shows the fuel efficiency curves of these generator setssuperimposed on histogram plots of the drawworks' power draw with andwithout the flywheel system. The efficiency curves show that a generatorset is most efficient when operating near its maximum power rating. Thehistograms are a measurement of the relative amount of time that thedrawworks is demanding a certain amount of power. When unaided by theflywheel device, large amounts of power (up to 7500 KW) are drawn forshort periods of time, but the overall power requirement is very low.With the flywheel-equipped drawworks of the present invention, the powerusage is much more consistent (approximately 700 KW). The consistentpower draw and the reduced average load results in a 75% lower fuelconsumption with only a single genset online instead of the usual six.The reduced fuel use and operational range of the engine/generator setswill result in significantly lower emissions.

As can be seen in FIG. 8, without the flywheel system of the presentinvention fuel consumption greatly exceeds the fuel consumption of thepresent invention by an order of magnitude.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof. Various changes in the details ofthe illustrated construction can be made within the scope of theappended claims without departing from the true spirit of the invention.The present invention should only be limited by the following claims andtheir legal equivalents.

1-10. (canceled)
 11. A process of managing energy consumption in adrawworks, the drawworks having a winch drum with a wire ropetherearound, the winch drum extending around a sheave assembly andconnected to a drill string, the process comprising: rotating the winchdrum so as to pay out the wire rope and to lower the drill stringdownwardly; transferring the energy from the lowered drill string to aflywheel so as to rotate said flywheel; and transferring energy from therotating flywheel to another location on the drawworks.
 12. The processof claim 11, said step of transferring energy from the rotating flywheelcomprising: transferring energy from the rotating flywheel to the winchdrum; and raising the drill string by paying in the wire rope over thewinch drum.
 13. The process of claim 11, further comprising: connectingan AC motor to the winch drum so as to apply rotational forces to thewinch drum; and connecting another AC motor to the flywheel.
 14. Theprocess of claim 13, said step of transferring energy from the lowereddrill string comprising: transferring energy from the AC motor connectedto the winch drum to said another AC motor connected to the flywheel.15. The process of claim 13, said step of connecting an AC motorcomprising: coupling said winch drum to a first AC motor by a shaft onone side of said winch drum; and coupling said winch drum to a second ACmotor by said shaft on an opposite side of said winch drum.
 16. Theprocess of claim 11, said another location being a utility system on thedrawworks.
 17. The process of claim 11, said drawworks being located ona drill ship, said another location being an engine associated with athruster of the drill ship.